1. Field of the Invention
The invention relates in general to a navigation equipment for a moving vehicle which utilizes a magnetic field probe for obtaining measured values for the direction of travel and uses a distance travel meter for acquiring measured values for the travel distance and further uses a plat memory and a digitized map of a region in a coordinate system.
2. Description of the Prior Art
These are Navigation equipment which identify the current position of a vehicle from measured directional values obtained with a magnetic field probe and use a travel distance meter. From direction and the travel distance are produced vectors which are coupled to each other so as to form a chain of vectors the end point of which represents the current position. Due to magnetic disturbances and other factors errors result which can become very large during long journeys. Positions indicated may be in error particularly in urban areas where the magnetic disturbance frequently occurs and wherein there is an increased demand of precision due to the dense street network.
So as to correct these position errors, it is assumed that a vehicle will normally move only on streets and that all identified positions which do not lie on the street are thus incorrect.
For this purpose, navigation equipment contains a plat memory in which a street map is stored in digitized form so as to compare the measured position with such street map and to correct the actual position. A navigation equipment comprising a digitized city map is described, for example, in the publication LOLA final report ("Experimentalstudie uber die Moglichkeit der Lokalisierung von Landfahrzeugen LOLA, Paragraph 32). The individual streets of the street map are thus each defined by a polygonal drawing composed of at least two pairs of coordinate points. The more pairs of coordinate points which are required for the description of a street defines in greater detail the curved course of a street or how irregular it is. A further pair of coordinate points identifies each street junction or intersection and it is important first to mark the beginning and end of the street and second to determine the three dimensional arrangement of the streets relative to each other.
This known navigational equipment checks whether a measured positional indication of the vehicle lies on one of the polygonal plats of the course of a large city. When deviations occur, it calculates that point on the street lying closes to the position indication and processes this point in continued navigation. The memory space required for such a digital street map, for example, for the plat of a large city is in the order of a few hundred kilobytes.
The memory is practically completely searched at every comparison and every correction of a positional indication. The shortest distance between the positional indication and the connecting line between the individual pairs of coordinate points must be obtained and this requires a considerable cost outlay for the equipment.